In order to prevent toxic materials in a ground-level or above-ground dump from leaching into the water table underneath the dump, it is known to install under the dump a leach field from which liquids can be drawn, to prevent them from working themselves deeper into the water table. Such a system does not provide a high degree of protection, as liquid can leak past the leach-field pipes. Furthermore such pipes must normally be installed before the dump is made, that is they cannot be installed readily under an existing dump.
It is also known, for example from German patent document 3,617,956 filed May 28, 1986 by Hans Richter to isolate an existing dump by driving under it a succession of tunnels. As each tunnel is driven, a liquid-stopping membrane and/or drainage pipes are installed. The area behind the driving machine is filled with stowing material derived from the face cutter of the machine to prevent caveins, and any excess such material, which may be stone, earth, or clay depending on the ground composition, is conveyed to the surface. Such a procedure is extremely complex an expensive.
In another known system two slit tunnels are driven directly under the dump by longwall-mining techniques. This forms a basically horizontal barrier directly underneath the dump that can be provided with a vapor barrier and/or a drainage system. Horizontally lateral leaching of pollutants in the dump can still take place, unless expensive vertical slit shafts are subsequently cut to join with the outer edges of the horizontal slit and thereby fully isolate the dump.